1. Field of the Invention
The present invention relates to a communication system in which plural terminals that are connected to each other through transmission paths operate in clock synchronization with a global time which is used on a network formed of the transmission paths.
2. Description of the Related Art
In a communication system for transmitting and receiving frames among the plural terminals through the transmission paths, in particular, a communication system employed in a control device of an in-vehicle device or a production line, an attention has been paid to a time trigger system (hereinafter referred to as “TT system”) which manages a frame transmission timing and a setting change timing by a time.
As a known TT system, there is a FlexRay (registered trademark) which is used in the network of an in-vehicle device. In this system, in the case where clock synchronization is established with respect to the global time that is used on the network to attempt the integration of the terminals into the network, the clock synchronization is conducted by a receiving time of a frame which is called “startup frame” that is transmitted from one or more terminals which have been already integrated into the network, and an estimated receiving time of the startup frame which is calculated from the communication schedule by a terminal that conducts the clock synchronization with the network (for example, refer to “FlexRay Communication System Protocol Specification Ver. 2.1”).
In the above FlexRay, in the case where clock synchronization is attempted with respect to the global time that is used on the network, an estimated reception time of the startup frame is obtained by an initial communication cycle, the reception time of the startup frame is obtained by a subsequent communication cycle. Then, a local time is initialized by the reception time and the estimated reception time. Further, when the local time has been completely initialized in a startup phase, an initial clock is adjusted by the reception time and the estimated reception time of the startup frame in two subsequent continuous communication cycles. Incidentally, in order to improve the transmission efficiency, a demand has been made to further shorten a period of time required to establish clock synchronization.